1. Field of the Invention
The present invention relates generally to a method for charging a nickel-hydrogen battery, and, more particularly, to an improved method for the xe2x80x9ccold chargingxe2x80x9d of nickel-hydrogen batteries as typically used in a spacecraft, by providing a closed loop control for charge control of the battery to minimize battery heat dissipation and spacecraft power requirements, and to maximize battery charge capacity.
2. Description of the Prior Art
It is well recognized that optimum performance for nickel hydrogen spacecraft batteries requires that their temperature during charging be controlled in the range of xe2x88x9220xc2x0 C. to +10xc2x0 C. Higher temperatures generally lead to lowered stored energy capacity due to the early on-set of the parasitic electrolysis reaction, e.g.,
20Hxe2x88x92=xc2xdO2+H2O+2exe2x88x92xe2x80x83xe2x80x83(1)
Reaction (1) has been recognized in the prior art to reduce capacity as the operating temperature during charging of the battery is increased. In U.S. Pat. No. 5,395,706 there is described information about the recognition of critical temperatures for charging a Nixe2x80x94H2 battery. The content of U.S. Pat. 5,395,706 is incorporated by reference into this application. The recognition is that the critical temperature is that at which the battery is recharged and reaction (1) competes with the normal recharge reaction, e.g.,
xe2x80x83Ni (OH)2+OHxe2x88x92=NiOOH+H2O+exe2x88x92xe2x80x83xe2x80x83(2)
The ""706 patent provides for the basis to optimally charge nickel-hydrogen batteries xe2x80x9cat a temperature T1 in the range of xe2x88x9210xc2x0 C. down to xe2x88x9230xc2x0 C., which is lower than a temperature T2, in the range of -xe2x88x9210xc2x0 C. up to +5xc2x0 C. at which discharge customarily begins.xe2x80x9d Lower temperatures on the other hand have been found to lead to batteries which fail to operate. The reasons for this failure, prior to the invention, described in U.S. Pat. No. 5,395,706, were unclear as the freezing point of the electrolyte (xe2x88x9261xc2x0 C.) is well below the minimum operation temperature. The second key to the invention in U.S. Pat. No. 5,395,706 is the recognition that the lower temperature limit is due to a limitation in the ability of the battery to sustain high rate discharge required in satellites situated in a geosynchronous orbit (e.g. xcx9cC/1.5) as opposed to the lower rate recharge (e.g. xcx9cC/10)-where xe2x80x9cCxe2x80x9d is a baseline, or nameplate, capacity of the battery.
A typical example of the prior art as it relates to charging and discharging nickel type batteries is U.S. Pat. No. 4,680,241 to Dyer. The Dyer patent concerns a method for partially or fully restoring the lost capacities of nickel batteries. In this instance, a nickel battery is cycled at least ten (10) times, with each cycle including a discharging step during which the capacity achieved at the end of the previous cycle is reduced by at least five (5) percent, and a charging step. The charging rate employed during the charging step is greater than about C/10 per hour. Moreover, while the ratio of the amount of charge delivered to the battery during the charging step of each cycle to the amount of charge withdrawn from the battery during the previous cycle is greater than one, this ratio is chosen so that the temperature of the electrolyte of the battery does not exceed about 30xc2x0 C.
The present invention adds to the prior art a method for the prevention of overcharging which leads to lower battery charge capacity and an inefficient use of spacecraft power. Programming requirements are much simpler for the present invention than that described by the prior art. Battery recharge is adjusted to account for battery recharge performance in a closed loop control as opposed to the prior art which is an open loop charge control. The method according to the present invention lends itself to easy application to recharging a battery after all battery discharges, including those caused by nominal earth eclipse periods, as experienced by spacecraft, and non-nominal discharges such as in the case of lunar eclipses, spacecraft emergencies, spacecraft orbital maneuvers, etc.
The features of the present invention provide for a closed loop feed back control of battery charging based upon battery heat dissipation through direct measurement of battery temperature (and/or direct measurement of battery heater duty cycles or battery strain gage data which is a direct measurement of battery pressure which is itself an indirect measurement of temperature at a given state of charge). The features of the present invention allows for automated battery recharge in eclipse and non-eclipse discharge cycles (such as orbit raising, lunar eclipse, and emergency operations).
To achieve the various advantages described herein in accordance with the purpose of this invention, the inventive features as embodied by the present invention include a method of charging a nickel-hydrogen battery which includes a positive electrode, a negative electrode, and an electrolyte, after the battery has been subject to discharge, comprising the steps of: (a) subjecting the battery to a high charge rate (e.g. about C/10 to C/15-wherein xe2x80x9cCxe2x80x9d is the normal full baseline or nameplate capacity of the battery) for a period until the battery temperature increases at least a set xcex94T1 (typically about 2xc2x0 C.) over the local minimum battery temperature i.e., the lowest temperature, during this initial charge, (b) switching the battery charge level to a lower rate of charge than in step (a) for a period until the battery temperature increases at least xcex94T2 (typically the same as xcex94T1 i.e., 2xc2x0 C.) over the local minimum battery temperature; (c) again switching the battery charge level to a minimum charge rate of approximately C/100 up and until about 30 to about 60 minutes before the next scheduled eclipse and thereafter; (d) switching the battery to a pulse charge equivalent to the high charge rate of step (a) until the next eclipse begins, or switching back to the minimum charge rate in step (c) after a maximum of 60 minutes of charge during step (d), i.e. no further battery discharge commences. Thus, the battery charge rate is closed loop controlled by changes of the battery temperature over time, i.e. temperature derivative.
Furthermore, in accordance with the features of the present invention a method of charging a nickel-hydrogen battery which includes a positive electrode, a negative electrode, and an electrolyte, comprises the steps of: (a) first subjecting the battery to a relatively high charge rate of about C/15 for a period until the battery temperature increases about 2xc2x0 C. over the local minimum battery temperature; (b) switching the battery charge level to a medium level of charge rate of about C/30, for a period until the battery temperature increases about 2xc2x0 C. over the local minimum battery temperature; (c) again switching the battery charge level to a relatively low level of charge of about C/100 for a period up to about 30 to 60 minutes before the next scheduled eclipse; and (d) switching the battery to a pulse charge equivalent to high charge rate of step (a).
According to the features of the present invention, a primary object of the present invention is to obtain an improved unique method for autonomously increasing the capacity of a nickel-hydrogen battery. Due to the unique simplified method for changing, the method can be performed manually, i.e. by ground command, in the event of a failure or absence of the autonomous means within the spacecraft.
Other objects of the present invention are for an improved method for the xe2x80x9ccold chargingxe2x80x9d of nickel-hydrogen batteries include providing a closed loop control for charge control of the battery to minimize battery heat dissipation and spacecraft power requirements.